The present invention relates generally to plasma arc welding, and, more specifically, to plasma arc welding or cladding underwater using a metal filler powder.
In one type of underwater welding, a filler metal in the form of a powder must be suitably delivered dry to a welding torch for welding or cladding a metal workpiece underwater. In one welding technique being developed, an Underwater Plasma Transferred Arc (UPTA) is used for cladding certain components of a Boiling Water Reactor (BWR) for enhancing the strength thereof.
In plasma arc welding, an electrical arc is formed in the presence of an ionizing gas to form a hot plasma which is used to melt the surface of the workpiece and upon which a filler powder is clad. A typical plasma torch includes a center electrode in a housing defining a second electrode between which the arc is formed. The ionizing gas is channeled through the torch and out a central nozzle orifice surrounding the center electrode's tip for forming the plasma. In a plasma transferred arc torch, the workpiece itself defines the second electrode, or ground, instead of the torch housing, and the arc is formed between the electrode and the workpiece. In either embodiment, the plasma generates sufficient heat for locally melting the workpiece and filler powder for cladding the workpiece.
The cladding process may be effected underwater by temporarily excluding or removing water from the immediate vicinity of the plasma arc so that the powder may be melted atop the developed melt pool which is quench cooled as the torch is carried along a welding path.
The water is temporarily excluded from the vicinity of the plasma torch by introducing a pressurized shielding gas inside a surrounding skirt for displacing the water therefrom. The pressure of the shielding gas must be suitably higher than the hydrostatic pressure of the water at the particular depth for expelling the water from the skirt surrounding the torch nozzle. Correspondingly, the filler powder is delivered to the torch using a suitable carrier gas which also must be at a suitably high pressure and flowrate for overcoming the hydrostatic pressure of the water at the specific depth.
In a typical plasma torch, a pair of diametrically opposite powder injection ports straddle the nozzle orifice from which the plasma arc is generated. The powder ports are simply tubular conduits extending through the nozzle and discharge radially outwardly of the nozzle orifice. As the depth underwater of the workpiece is increased, the hydrostatic pressure increases which in turn requires an increase in the pressure of the powder carrier gas which adversely affects the plasma and welding process as the powder is ejected. Testing has shown that high flow carrier gas causes the plasma arc to distort into two portions, which in turn results in formation of a double bead weld. This prevents the formation of a uniform weld or cladding layer and is therefore undesirable.
In U.S. Pat. No. 5,690,844, assigned to the present assignee, this problem has been addressed and significantly alleviated using a remote torch feed hopper in which the powder is delivered with a high flow and high pressure carrier gas, with the gas being vented prior to final delivery of the powder to the nozzle. In this way, the powder may be carried to substantial depths underwater and vented to minimize adverse affect when injected into plasma arc.
However, the vented carrier gas may entrain some of the powder for which additional components such as a powder collector, water vapor filter, and flowmeter are provided. Furthermore, performance of the remote hopper varies as the hydrostatic water pressure varies.
Accordingly, it is desired to provide an improved plasma torch feed hopper having fewer components and operable at variable hydrostatic pressure.